Rare earth magnet is based on intermetallic compound R2T14B, thereinto, R is rare earth element, T is iron or transition metal element to replace iron or part of iron, B is boron, it is known as king of the magnet with excellent magnetic properties, the max magnetic energy product is ten times higher than that of the ferrite magnet, besides, the rare earth magnet has well machining property, the operation temperature can reach 200° C., it is hard, stable, with well cost performance and wide applicability.
There are two types of rare earth magnets depending on the manufacturing method: sintered magnet and bonded magnet. Sintered magnet has wide applications. In existing known technology, sintering method of rare earth magnet is normally performed as follows: raw material preparing→melting→casting→hydrogen decrepitaiton→micro grinding→pressing under magnetic field→sintering→heat treatment→magnetic property evaluation→oxygen content evaluation of the sintered magnet.
The process of pressing the sintered rare earth magnet under magnetic field is applied with a forming method called two-stage digestion process, which is widely used in the early time, the method is applied with a simple module mold that the magnet is formed under a low pressure (about 0.2 ton/cm2) magnetic filed (the first stage of process), taken out manually and packaged, then it is formed by isostatic pressing under oil high pressure (1.4 ton/cm2) (the second stage of process), as the isostatic pressing forming is using manual method, it takes long time in this process, oil pollution after forming and oxidation during transportation will cause quality management problems of the products.
To solve above problems, recently used is one-stage process, that is to say, it is applied with a transverse magnetic field orientation type—one-stage automatic pressing machine. Compared to the two-stage digestion process, one-stage forming (the maximum forming pressure is about 0.8 ton/cm2) has weak forming pressure, broken, corner-breakage or crake frequently happen to the sintered magnet. Besides, during one-stage forming, the initial pressure raises to 0.6˜0.80.8 ton/cm2, compared to two-stage forming (the pressure of the first stage is 0.2 ton/cm2), as time goes on, the degree of orientation is worse and worse, leading to decreasing of degree of orientation and low Br, (BH)max of the products.
Disclosed in U.S. Pat. No. 6,461,565 is a transverse magnetic field orientation type—one-stage automatic pressing machine, however, due to the limited technology at that time, fully sealing technology is not grade, the oxygen content during forming is controlled below 10000 ppm, spark happens during forming, so that this invention's main improvement point is to prevent unqualified products due to burning or heating of green compacts, researchers found that, controlling the compacting temperature below 5˜30° C. and the humidity in 40˜65% can prevent rapid oxidation due to burning or heating. In the specification of the U.S. Pat. No. 6,461,565, automatic mechanical operation device needs frequency maintenance, which can not ensure leakproofness, thus making oxidation more easily happens. During forming, the oxygen content and the relative humidity are high, so that this method obtains sintered magnet with oxygen content over 2900 ppm, so that sintered products with lower oxygen content and better magnetic property can not be obtained.
With the development of the technology, existing nitrogen or inert jet stream replacement technology can simply realize fully sealing to obtain sintered magnet with lower oxygen content. Therefore, the producers focus on the relative technology of transverse magnetic field orientation type—one-stage automatic pressing machine based on fully sealing technology, however, broken, corner-breakage, crack and other bad problems frequently happen to the sintered magnet when adding nitrogen or inert gas to the fully sealing pressing machine. The reasons are that the nitrogen or inert gas has very low oxygen content and very low relative humidity, for example, pure nitrogen contains almost none of water when in low dew point of below −60° C., the atmosphere of the pressing machine is situated in an ultra-dry condition with relative humidity below 3%. This ultra-dry condition easily builds up static, the static electricity makes the powder produce strong electrostatic repulsion leading to bad forming property, it also leads to decreasing of degree of orientation and the Br, (BH)max. The reason is that if the surface of the powder has no oxygen and water, it will be solid like metal binding, the frictional resistance of the powder increases, leading to decreasing of degree of orientation.
Besides, when adding inert gas into the atmosphere of the fully sealing pressing machine, if forming in a low oxygen content and low humidity, abnormal grain growth (AGG) or reduced coercive force easily happens. Moreover, the low oxygen content in sintered magnet may cause failed HAST experimental result. The reason is that, no oxidant exists in the ultra-low oxygen content and ultra-low humidity condition, no-oxidation metal Nd increases, abnormal grain growth (AGG), reduced coercive force and failed HAST experimental result easily happen.